As of AppleLLVM compiler version 5.1 (clang-502) and later, the optimization level -O4 no longer implies link time optimization (LTO). In order to build with LTO explicitly use the -flto option in addition to the optimization level flag. (15633276)
The AppleLLVM compiler in Xcode 5.1 treats unrecognized command-line options as errors. This issue has been seen when building both Python native extensions and Ruby Gems, where some invalid compiler options are currently specified.
Projects using invalid compiler options will need to be changed to remove those options. To help ease that transition, the compiler will temporarily accept an option to downgrade the error to a warning:

Note: This option will not be supported in the future.
To workaround this issue, set the ARCHFLAGS environment variable to downgrade the error to a warning. For example, you can install a Python native extension with:
$ ARCHFLAGS=-Wno-error=unused-command-line-argument-hard-error-in-future easy_install ExtensionName